Currently, there are no known algorithms for the detection of machine printed characters that were printed on a dot matrix or ink jet printer.
U.S. Pat. No. 4,547,811 is directed to a method and apparatus for gray level signal processing. A gray level signal processing method determines whether an image signal derived from an original document belongs to the character image area which needs high resolution or the continuous-tone image area which needs gray level reproduction such as for photographs. The process comprises the steps of: Dividing the picture image into blocks, each composed of m.times.n picture elements. The maximum and minimum optical density levels are detected in each block. Next, the difference between the maximum and minimum optical density levels is calculated. The image area to which the block belongs is determined by the difference value previously calculated. The optimum quantization processing method is selected according to the optical density level of each level.
U.S. Pat. No. 4,741,046 to Matsunawa operates on binary image data, whether processing the entire picture or blocks of the picture, in order to determine if the image is continuous tone, halftone or a line picture. A histogram of the image data is used to quantify the background level and select the appropriate threshold.